Professor Ranjan Gupta

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    Scattering Properties and Composition of Cometary Dust
    (Astrophysics and Space Science, 2005-04-11) Gupta, Ranjan; Vaidya, D.B.; Bobbie, J.S; et.al
    Composition of the Comet dust obtained by the dust impact analyzer on the Halley probes indicated that the comet dust is a mixture of silicate and carbonaceous material. The collected interplanetary dust particles (IDP’s) are fluffy and composite, having grains of several different types stuck together. Using discrete dipole approximation (DDA) we study the scattering properties of composite grains. In particular, we study the angular distribution of the scattered intensity and linear polarization of composite grains.We assume that the composite grains are made up of a host silicate sphere/spheroid with the inclusions of graphite. Results of our calculations on the composite grains show that the angle of maximum polarization shifts, and the degree of polarization varies with the volume fraction of the inclusions.We use these results on the composite grains to interpret the observed scattering in cometary dust.
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    Scattering Properties and Composition of Cometary Dust
    (Astrophysics and Space Science, 2005-04-11) Gupta, Ranjan; Vaidya, D.B.; Bobbie, J.S; et.al
    Composition of the Comet dust obtained by the dust impact analyzer on the Halley probes indicated that the comet dust is a mixture of silicate and carbonaceous material. The collected interplanetary dust particles (IDP’s) are fluffy and composite, having grains of several different types stuck together. Using discrete dipole approximation (DDA) we study the scattering properties of composite grains. In particular, we study the angular distribution of the scattered intensity and linear polarization of composite grains.We assume that the composite grains are made up of a host silicate sphere/spheroid with the inclusions of graphite. Results of our calculations on the composite grains show that the angle of maximum polarization shifts, and the degree of polarization varies with the volume fraction of the inclusions.We use these results on the composite
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    Imaging polarimetry of some selected dark clouds?
    (Astron. Astrophys. Suppl. Ser., 2000-01-11) Sen, A.K.; Gupta, Ranjan; Ramaprakash, A.N; et al.
    A set of eight Bok Globules CB3, CB25, CB39, CB52, CB54, CB58, CB62 and CB246 were observed polarimetrically in white light, using our Imaging Polarimeter (IMPOL), from the 1.2 m IR telescope at Mount Abu, India. The observations were carried out on di erent nights during the period December 1997 and April 1998. The CCD images obtained from the instrument (IMPOL) were analyzed, to produce polarization map of the Bok Globules. The stars in the eld, which are mostly background to the cloud show typically 2% of linear polarization. Clouds which are less dynamic (having 12CO line widths V < 2:5 km s−1), in general show slightly better alignment of polarization vectors with the projected direction of galactic plane. On the other hand the more dynamic group of clouds, has the polarization vectors more scattered and poorly aligned with the projected direction of the galactic plane. However one of the clouds observed CB58 does not follow this trend very well.
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    An Imaging Polarimeter(IMPOL) for multi-wavelength observations
    (Astron. Astrophys. Suppl. Ser, 2008-02-02) Ramaprakash, A.N; Gupta, Ranjan; Sen, A.K.; et.al
    Taking advantage of the advances in array detector technology, an imaging polarimeter (IMPOL) has been constructed for measuring linear polarization in the wavelength band from 400-800 nm. It makes use of a Wollaston prism as the analyser to measure simultaneously the two orthogonal polarization components that define a Stoke’s parameter. An achromatic half-wave plate is used to rotate the plane of polarization with respect to the axis of the analyser so that the second Stoke’s parameter also can be determined. With a field of view correponding to about 30 × 30 mm2 for a / 1.2 m, f/13 telescope, a sensitive, liquid-N2 cooled CCD camera as the detector and a built-in acquisition and guidance unit, the instrument can be used for studying stellar fields or extended objects with an angular resolution of ∼2′′ . The instrumental polarization is less than 0.05% and the accuracies of measurement are primarily limited by photon noise for typical observations.